Lesbians are just as likely to get sexually transmitted diseases as heterosexual women, new research suggests.

And they are far more likely to be drug users and to have had sex with homosexual men than their heterosexual counterparts.

Consistent with this at-risk behaviour, they are nearly eight times more likely to be infected with the hepatitis C virus, Australian scientists say.

Doctors at Sydney Sexual Health Centre found that bacterial vaginosis was more prevalent in women who had sex with other women, while infections such as herpes and genital warts were common in both groups.

Both groups had low and similar rates of gonorrhoea and chlamydia.

Their findings, from 2800 women, confound previous assumptions that lesbian women were at low risk of contracting sexually transmitted infections.

It is very important that, if a women describes herself as lesbian, we find out about whether she also has sex with men

Prof Michael Adler

The authors said that "a woman's sexual identity is not an accurate predictor of behaviour, with a high proportion of 'lesbian' women reporting sex with (often high risk) men".

What they found was the almost all the women who had sex with other women had at some time also had sex with men - only 7% had not had sexual contact with a male.

Women who had sex with women were more than three times more likely to report having had sex with a homosexual or bisexual man.

In fact, almost one in ten of the women who had sex with other women reported having had more than 50 male sexual partners.

Drug injecting

They were eight times more likely to be hepatitis C positive, and eight times more likely to inject drugs than women who had sex exclusively with men.

Almost a quarter of women who had sex with women said they had injected drugs at some time.

In an editorial accompanying the study, published in the journal Sexually Transmitted Infections, Dr Jeanne Marrazzo of the University of Washington highlighted the "alarming prevalence of HIV-related risk behaviours in women who have sex with women".

She said previous assumptions about low risk in gay women were "informed by disinterest or by a lack of willingness to believe that the area is worthy of further study; at worst, homophobia and sexism contribute to these views".

Professor Michael Adler, Head of Reproductive Health at University College Hospital in London, said the study reinforced the importance of taking a good sexual history from people visiting STD clinics.

"It is very important that, if a women describes herself as lesbian, we find out about whether she also has sex with men or whether there is other behaviour that is putting her at risk.

"Women who are exclusively lesbian are at low risk of sexually transmitted infections, but sex with men puts her at the same risk as all other women."